Field of the Invention
The invention relates to a semiconductor electronic device primarily used for high-power and/or high-frequency electric/electronic circuit. More specifically, the invention relates to transistors formed of group III nitride semiconductor.
Description of the Existing Technology
(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III nitride alloys are the key semiconductor material for various electronic devices such as power switching transistors. Despite the fact that the maximum performance of GaN theoretically predicted with Baliga's Figure of Merit (BFOM) exceeds that of silicon carbide (SiC) by approximately 5-fold, the lack of low-cost and low-defect GaN substrates impedes development of GaN-based power switching transistors having their full potential. Currently, the majority of GaN-based devices are fabricated using a group III nitride film grown heteroepitaxially on a heterogeneous substrate, such as silicon, SiC and sapphire. However, heteroepitaxial growth of group III nitride results in highly defected or even cracked films. Typical defects in group III nitride heteroepitaxial films are threading dislocations at the level of 109 cm−2 along the growth direction. Because threading dislocations and/or cracks propagate vertically through a substrate, these defects can become current leakage paths when high-voltage is applied vertically (i.e. along the substrate's growth direction, typically described as the substrate's thickness).
Therefore, at this moment, GaN-based electronic devices are practically limited to horizontal devices such as high-electron mobility transistors (HEMT), which utilize current flow along the lateral direction near a substrate's major surface. Since the electric current passes through a thin film in such horizontal devices, a horizontal device requires a large major-surface area to realize high-current (i.e. high-power) devices. In addition, all contacts are located on one side of the device, which causes device size to be larger than a vertical configuration. Due to these limitations, it is quite challenging to attain high-power devices in horizontal configuration of group III nitride semiconductors.
To overcome the issues in horizontal group III nitride devices, people have started to develop vertical type electronic devices using GaN substrates. Vertical high-power switching devices require normally-off operation, low-series resistance, high-breakdown voltage, fast switching speed, high efficiency and low cost. However, people have not demonstrated viable vertical high-power transistors using group III-nitride semiconductors due to many technical challenges such as difficulties in obtaining low-cost, low-defect substrates, growing high-purity drift layers with accurate control of carrier concentration, and/or fabricating high-quality buried portion of group III-nitride crystals for controlling current paths. To achieve commercially viable high-power vertical devices with group III-nitride semiconductors, a selection of substrate, design of device structures and a selection of fabrication method must be carefully considered.